1. Field of the Invention
The present invention relates to an image processing method and apparatus for quantizing input image data into data having levels which are smaller in number than input levels of the input image data and, more particularly, to an image processing method and apparatus for performing a quantizing process such that image data is adapted to an output device in which an output area of one pixel changes depending on the position of the pixel.
2. Description of the Related Art
In order to reproduce a variable-density image by a printer or a display which performs a binary display by black and white, an area gradation method which expresses the variable-density image by forming a pseudo digital half-tone image has been conventionally used. The area gradation method is a method for changing the ratio of black in a neighborhood image to reproduce a gradation image. As this method, a dither method in which a variable-density image u(x,y) of an original image is compared with a threshold value T calculated by a predetermined rule or an error diffusion method in which an error between an input variable-density image value and an output half-tone image is diffused to a pixel which is not scanned is practically used.
In a color image, the following technique is conventionally known. That is, an input color image is subjected to a half-tone process by using the dither method or the error diffusion method to obtain low-bit data (e.g., 1-bit binary data), and a full-color image is formed by using a binary printer and a binary display device.
However, the conventional dither method or the conventional error diffusion method is designed on the assumption that an output device has the same gradation expression capability for all pixels (all the pixels have the same maximum luminance, the same number of bits, and the same color expression capability). For this reason, each of these method has a drawback that an output device which has different maximum luminances, the different numbers of bits, and different color expression capabilities for respective pixels cannot be used. In addition, in a device such as a liquid-crystal display in which the number of pixels is fixed, when the number of pixels of display data and the number of pixels of the device are different from each other, a half-tone process must be performed after resolution conversion is performed by an interpolation process.